Efficiency of Geothermal Ground Improvement System

ABSTRACT

Provided herein are apparatus, systems and methods for use in ground construction. New ground improvement methods have been developed using pervious concrete piles, the piles having material properties of the pervious concrete, as well as positive response to different vertical loads. The methods produce piles that are superior to aggregate piles in most aspects, and that can be utilized in construction in a wider variety of soils and applications, including in geothermal heat exchange (energy pile) systems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No. 61/676,446 filed Jul. 27, 2012.

GOVERNMENT LICENSE STATEMENT OF INTEREST

This invention was made with government support under Grant/Contract No. 0927743 awarded by the National Science Foundation. The government has certain rights in the invention

BACKGROUND OF THE INVENTION

Numerous structures and highway facilities, including embankments and bridges, are often constructed on poor soils (i.e., soft or loose soils as well as organic/peat soils). In order to facilitate construction, achieve allowable settlements, and avoid failures, poor soils are often improved using ground improvement technologies. A common ground improvement technique involves using permeable granular piers (also known as “aggregate piers”) that include sand compaction piles, stone columns and rammed aggregate piers, to improve soil strength and provide a drainage path. The use of permeable granular piers increases the time rate of consolidation, reduces liquefaction potential, improves bearing capacity and reduces settlement (Barksdale and Bachus, 1983; Mitchell, 1981; Aboshi and Suematsu, 1985; Bergado, 1994; Baez, 1995; Terashi and Juran, 2000; and Okamure et al., 2006). However, when compared to other pile types (i.e., steel and concrete piles), the strength and stiffness of granular piers are lower and depend on the properties of the surrounding soil. Therefore, granular piers have limited use in very soft clays and silts, and organic and peat soils.

Many construction methods, including vibro-composer, vibro-compaction, vibro-replacement, impact, and ramming compaction, are used to install permeable granular piers (Aboshi et al., 1979; Mitchell, 1981; Barksdale and Bachus, 1983; Bergado et al., 1994; Moseley and Kirsch, 2004; White and Suleiman, 2004; and Geopier Foundation Company, 2012). These construction methods alter the soil stresses resulting in lateral consolidation and/or compaction of the surrounding soil (Handy, 2001; Basu et al., 2011; and Lundberg et al., 2013). However, the effects of granular piers construction on soil stresses and displacement have been investigated by few researchers (Shublaq, 1992; Hunt, et al., 2002; Lee, et al., 2004; Suleiman and White, 2006; Elshazly et al., 2008; Chen et al., 2009; Yi et al., 2010; Dijkstra, et al., 2010; and Thompson and Suleiman, 2010). Different approaches have been used to evaluate the effects of granular pier installation on surrounding soils, including cavity expansion analysis, numerical modeling methods, cone penetration tests before and after installation, and shear wave velocity measurements. None of these approaches, however, directly and simultaneously measured the changes of soil stresses and soil movement during pile installation. Lundberg et al. (2013) reported that there is a lack of direct combined measurements of soil stresses and movement surrounding displacement piles in general; a knowledge gap that is partially addressed as part of this paper for the installation method used to construct pervious concrete ground improvement piles. Further experimental tests measuring the effects of the installation method on the surrounding soil, along with analytical modeling, are being performed by the research team.

Permeable granular piers are used in ground construction to increase the time rate of consolidation, reduce liquefaction potential, improve bearing capacity, and reduce settlement. However, the behavior of granular piers varies widely, and depends on the confinement provided by surrounding soil, which limits their use in very soft clays and silts, and organic and peat soils.

For all the above reasons, there exists a need for piers and piles and methods of installation that provide the positive advantages of granular piers, while avoiding most or all of the disadvantages thereof. There further exists the need for new and improved methods to test the installation of pier and piles, and to adjust the installation to accomplish desired properties of pervious concrete piles while avoiding undesirable properties of the prior art piers, such as granular piers.

SUMMARY OF INVENTION

Provided herein are apparatus, systems and methods suitable for ground construction and ground improvement.

In an example, the apparatus, methods, and systems utilize new and inventive pervious concrete piles. In an embodiment, A method of installing a pervious concrete pile, the method comprising the steps of: a) providing a mandrel, the mandrel having a cone-shaped tip on a distal end thereof and an inlet opening on the opposite end thereof, a mandrel body connecting the inlet opening to the cone-shaped tip, the cone tip having means for opening and closing of the cone tip to allow material to pass from the inlet opening of the mandrel, through the mandrel body, and out of the con-shaped tip when the cone-shaped tip is opened; b) providing a vibration mechanism in vibrational contact with the mandrel; c) Inserting the mandrel into a geological formation at a preselected location; d) After installing the mandrel into the geological formation at a preselected location, delivering a selected amount and type of aggregate and concrete mixture into the inlet opening of the mandrel; e) Operating the vibration mechanism for a time sufficient to ensure delivery of the mixture to the cone-shaped tip of the mandrel; f) opening the cone-shaped tip of the mandrel to allow at least some of the mixture to escape from the tip to form a pervious concrete pile adjacent the exterior of the cone-shaped tip; and f) withdrawing the mandrel from the geologic formation at a rate sufficient to form a continuous pervious concrete pile within the geologic formation, the continuous pervious concrete pile having a porosity of between about 10% to about 40%, and a permeability coefficient of between about 0.01 to about 2 cm/sec.

In another embodiment, a geothermal heat exchange system is provided comprising at least one pervious concrete pile having a porosity of between about 10% to about 40%, and a permeability coefficient of between about 0.01 to about 2 cm/sec.

The new pervious concrete piles and methods of installation provide higher stiffness and strength, which are independent of surrounding soil confinement, while offering permeability comparable to granular piers. This provided ground improvement apparatus and methods improve the performance of different structures supported on the pervious concrete piles, even on otherwise undesirable soils.

The pervious concrete piles and associated methods of construction herein further provide the advantage of use in geothermal foundation applications (energy piles), such as in heat transfer systems to accomplish the heating and cooling of buildings and other surface-based structures that are supported by the piles. The pervious concrete piles can be installed as a closed-loop system similar to the ones currently used in other energy piles. However, an additional advantage over all known energy piles is that, given the permeability of the pervious concrete material used in the examples herein, the pervious concrete piles can also function as an open-loop system by interacting with groundwater that permeates the pervious concrete energy piles. Thus, the present inventive pervious concrete energy piles and methods combine the concept of open-loop and closed loop systems, unprecedented by any known system. Whether the piles are used as closed loop, open loop, or a combination of closed and open loops, the piles offer unprecedented advantages. For example, the permeability of the pervious concrete pile allow ground water to penetrate the pile, improving its heat transfer with surrounding soil and improving the efficiency of the system compared to other energy piles using normal concrete and closed loops.

These and other embodiments, examples, and details are provided in the accompanying specification, claims, abstract, and figures.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a table showing properties of three types of granular piers;

FIG. 1( a) is a photographic depiction of pervious concrete samples prepared by the methods described herein in accordance with the present invention;

FIG. 1( b) is a graph illustrating pervious concrete compressive strength and permeability samples prepared by the methods described herein in accordance with the present invention;

FIG. 1( c) is a graph illustrating gradation of aggregate used for casting test piles in examples herein, as well as for soil used in the soil box herein in accordance with the present invention;

FIG. 1( d) is a graph illustrating CD triaxial tests on sand samples in examples herein in accordance with the present invention;

FIG. 1( e) is a graph illustrating a diagram showing p′_(f)-q′_(f) for the peak stresses of soil samples in examples herein in accordance with the present invention.

FIG. 2( a) is a schematic drawing illustrating a top perspective view of the SSI facility and reaction frame system for vertical load tests in accordance with the present invention;

FIG. 2( b) is a photograph illustrating a side elevational view of the top box and reaction system of the SSI facility in accordance with the present invention;

FIGS. 3( a)-(e) are a series of photographs collectively depicting an example of the pervious pile installation method steps in an example herein; wherein:

FIG. 3( a) is a photograph illustrating an assembly of a cone, mandrel, and vibrator in accordance with the present invention;

FIG. 3( b) is a photograph illustrating an assembly of a bracing system assembled in the SSI facility top soil box in accordance with the present invention;

FIG. 3( c) is a photograph illustrating an assembly of the cone, mandrel, and vibrator of FIG. 3( a) in conjunction with the bracing system of FIG. 3( b), the mandrel being inserted and driven into the soil box in accordance with the present invention;

FIG. 3( d) is a photograph illustrating a step of casting a pile using the inserted and driven assembly of a cone, mandrel, and vibrator in accordance with the present invention;

FIG. 3( e) is a photograph illustrating the effect of opening the cone tip of FIG. 3( a) during the casting step of FIG. 3( d) vibrator in accordance with the present invention;

FIGS. 4( a)-(e) are a series of photographs collectively depicting an example of the vertical load test setup and results in example herein; wherein:

FIG. 4( a) is a photograph illustrating an exemplary assembly of a top soil box, bottom soil box, and reaction frame in accordance with the present invention;

FIG. 4( b) is a photograph illustrating an exemplary setup and method step of raining soil into a bottom soil box from an elevation of about 1.5 meters in accordance with the present invention.

FIG. 4( c) photograph illustrating an exemplary assembly and arrangement of SSAs used in experimental examples for forming piles in accordance with the present invention;

FIG. 4( d) is a photograph illustrating an exemplary assembly and arrangement of in-soil null pressure sensors used in experimental examples for forming piles in accordance with the present invention;

FIG. 4( e) is an enlarged view taken from FIG. 4( d) showing a null pressure sensor;

FIG. 5( a) is a schematic side cross-sectional view of the test SSI facility assembly used in performing experimental methods for Test Unit 4 to create experimental pile apparatus in accordance with the present invention;

FIG. 5( b) is a schematic top cross-sectional view of the test SSI facility assembly used in performing experimental methods for Test Unit 4 to create experimental pile apparatus in accordance with the present invention;

FIG. 6( a) is a graph depicting vertical load versus displacement for Test Units 1 and 2 for creating 76 mm diameter piles in accordance with the present invention;

FIG. 6( b) is a graph depicting vertical load versus displacement for Test Units 3 and 4 for creating 102 mm diameter piles in accordance with the present invention;

FIGS. 7( a)-(d) are a series of photographs collectively depicting the results of pervious pile installation methods in examples herein, wherein:

FIG. 7( a) is a photograph illustrating a result in Test unit 1, showing a bulging failure of the pile;

FIG. 7( b) is a photograph illustrating a result in Test unit 2, showing a pile assembly in accordance with the present invention in accordance with the present invention;

FIG. 7( c) is a photograph illustrating a result in Test unit 3, showing a pile assembly in accordance with the present invention;

FIG. 7( d) is a photograph illustrating a result in Test unit 4, showing a pile assembly in accordance with the present invention;

FIG. 8 is a graph illustrating a comparison of the force transferred along the length of piles as described in Test unit 3 and Test unit 4 for different loading stages, in accordance with the present invention;

FIG. 9( a) is a graph illustrating shear stress versus displacement curves (t-z curves) for the soil-pile interface calculated using the strain gauge measurements for Test units 3 and 4 for average depth below soil surface of 191 mm, in accordance with the present invention;

FIG. 9( b) is a graph illustrating shear stress versus displacement curves (t-z curves) for the soil-pile interface calculated using the strain gauge measurements for Test units 3 and 4 for average depth below soil surface of 635 mm, in accordance with the present invention;

FIG. 10 is a graph illustrating soil lateral displacement at 51 mm and at 152 mm from the surface of the pile measured using SSAs for Test unit 4, in accordance with the present invention;

FIG. 11( a) is a graph illustrating the effect of installation on the change of soil pressure showing a summary of pressure changes developed at the end of installation and during load testing, in accordance with the present invention;

FIG. 11( b) is a graph illustrating the effect of installation on the change of soil pressure showing details of changes of horizontal pressure measured using sensor 6 and changes of vertical pressure measured using sensor 3 focusing on the changes during installation, in accordance with the present invention.

DETAILED DESCRIPTION OF INVENTION

Provided herein are apparatus, systems and methods suitable for ground construction and ground improvement. In an example, the apparatus, methods, and systems utilize new and inventive pervious concrete piles. The new pervious concrete piles and methods of installation provide higher stiffness and strength, which are independent of surrounding soil confinement, while offering permeability comparable to granular piers. This provided ground improvement apparatus and methods improve the performance of different structures supported on the pervious concrete piles, even on otherwise undesirable soils.

The pervious concrete piles and associated methods of construction herein further provide the advantage of use in geothermal applications, such as in heat transfer systems to accomplish the heating and cooling of buildings and other surface-based structures that are supported by the piles, and produce enhanced heat transfer with surrounding soil. The pervious concrete piles and installation methods are compatible with the installation of closed-loop heat exchange pipes and systems, allowing for unprecedented heat exchange with the adjacent soil/earth along the entire pile length.

In this patent application, the material properties of pervious concrete, the developed installation method, and the vertical load response of pervious concrete and aggregate piles are presented, and the effects of the installation method on soil stresses and displacement are also discussed. For example, as further described herein, experiments were set up to prove that the inventive methods herein would produce novel pervious man-made pile structures having desirable properties and avoiding the problems inherent in prior art methods and resulting piers. In the exemplary experiments herein, four vertical load tests were performed on one granular pier and three pervious concrete piles. The experimental test results show that the ultimate load capacity of the pervious concrete pile was 4.4 times greater than that of an identical granular pier. In addition, the ultimate load capacity of a pervious concrete pile installed using the exemplary methods herein was 2.6 times greater than a precast pervious concrete pile. Other positive and unprecedented advantages of the exemplary installation methods herein, including creating non-uniform lateral soil displacement and increased vertical and horizontal soil stresses, are also discussed.

This research effort proposes the use of pervious concrete piles that can provide higher stiffness and strength that are independent of the surrounding soil properties, while offering permeability comparable to granular piers, to support structures and highway facilities constructed on poor soils. The goal of the present research was to develop an innovative ground improvement alternative that uses pervious concrete piles. This application focuses on: (1) presenting the material properties of pervious concrete and describing the developed installation method for pervious concrete piles; (2) comparing the response of pervious concrete and aggregate piles when subjected to vertical loading; (3) comparing the vertical loading response of precast pervious concrete pile with that of cast-in-place pervious pile constructed using the developed installation method; and (4) briefly discussing the effects of the installation method on soil stresses and displacement (or movement). In addition, the improvement of heat transfer when pervious concrete piles are used as energy piles will be discussed.

The method used to install granular piers affects their properties. Table 1 provides a comparison of published properties of sand compaction piles, stone columns and rammed aggregate piers.

For the range of design loads, the friction angle of sand compaction piles, stone columns and rammed aggregate piers ranges from 30-36°, 35-45° and 48-52°, respectively. White and Suleiman (2004) conducted triaxial tests on different types of compacted aggregates used in constructing granular piers and reported an average friction angle of 48.5° and a cohesion of 30 kPa. Table 1 also shows that the modulus of granular piers ranges from 25 to 120 MPa and the measured stress concentration ratios range from 1.5 to 10, while the permeability measured using laboratory and field tests ranges from 0.05 cm/sec. to 2.0 cm/sec.

Regardless of the construction method used, single isolated granular piers fail in bulging, shear, or punching (Barksdale and Bachus, 1983). For typical granular pier length to diameter (L/D) ratios, the most common failure mechanism is bulging, which is usually observed over a distance of 2 to 3 pier diameters below the soil surface (Barksdale and Bachus, 1983; Bergado et al., 1994). The ultimate vertical load capacity for a bulging failure mechanism of granular piers depends on the confinement provided by the surrounding soil (Hughes and Withers, 1974). For this reason, the use of granular piers is limited in poor soils, where minimum confinement is provided by the surrounding soil (Barksdale and Bachus, 1983; and Bergado et al., 1994). This research effort proposes an innovative ground improvement method using pervious concrete piles to overcome these limitations.

Pervious Concrete Material.

Pervious concrete is a special concrete product made primarily of a single-sized aggregate. Pervious concrete has been used in pavements to reduce storm water runoff quantities and perform initial water quality treatment by allowing water to penetrate through the surface. In the United States, pervious concrete is mainly used in pavement applications, including sidewalks, parking lots, tennis courts, pervious base layers under heavy duty pavements and low traffic density areas (Tennis et al., 2004; and Suleiman et al., 2011). Based on previous material studies (Kajio et al., 1998; Beeldens et al., 2003; Tennis, et al., 2004; Park and Tia, 2004; and Suleiman et al. 2006), pervious concrete material has a porosity ranging from 11% to 31%, a 28-day compressive strength between 5.5 MPa and 26.0 MPa and a permeability coefficient ranging from 0.25 to 0.54 cm/sec. Recent material tests performed by Kevern et al. (2008) indicated that the 28-day compressive strength of pervious concrete ranged from 17.0 MPa to 26.5 MPa and the permeability coefficient ranged from 0.02 to 1.03 cm/sec. Applicant incorporates each of the above published references as though fully set forth herein. Those references are available to one skilled in the art and are authoritative concerning the many types of pervious concrete, which are compatible for use with the present apparatus and methods.

To investigate the benefits of the proposed pervious concrete pile ground improvement method, four vertical load tests were conducted in a Soil-Structure Interaction (SSI) facility. The Test units included one aggregate pier (Test unit 1) and three pervious concrete piles (Test units 2, 3, and 4). All the Test units were installed in loose well-graded sand. Test units 1 and 2 were used to compare the behavior of an aggregate pier to a pervious concrete pile. Test units 3 (precast) and 4 (cast-in-place or installed) were used to evaluate the effects of the installation method on the behavior of pervious concrete piles subjected to vertical loading. The following sections of this paper focus on presenting the pervious concrete material properties; describing the developed installation method for pervious concrete piles; comparing the vertical loading response of pervious concrete and aggregate piles; comparing the vertical loading response of precast and installed pervious concrete piles; and briefly discussing the effects of the installation method on soil stresses and movement.

Material Properties

Pervious Concrete Properties

A series of pervious concrete mixtures were prepared in order to obtain an adequate compressive strength and permeability. Pervious concrete cylinder samples were tested to measure the porosity, permeability, compressive strength, elastic modulus, and split tensile strength. The compressive strength was determined using ASTM C39 (2009a), the permeability was measured using an in-house designed falling head permeameter, the porosity was measured using ASTM C1688 (2009b), the elastic modulus was measured using ASTM C469 (2009c) and the split tensile strength was measured using ASTM C496 (2009d). Several aggregate types, sizes and compaction (vibration) times were investigated. Mixtures were prepared using water/cement ratios (w/c) ranging from 0.21 to 0.27 and sand/aggregate ratios ranging from 5% to 11% using two mixing procedures and three different compaction times.

Mixtures that used a compaction (vibration) time of 10 seconds per layer during sample preparation resulted in an adequate compressive strength and permeability with no or minimal segregation of cement and aggregate (FIG. 1 a). FIG. 1 b summarizes the 28-day compressive strength and permeability coefficient results for pervious concrete mixtures that were prepared using a 10 second compaction time for different water/cement and sand/aggregate ratios, as a function of porosity. The results presented in FIG. 1 b indicate that the porosity ranged from 6% to 23% with the 28-day compressive strength ranging from 10.0 to 34.0 MPa and the permeability coefficient ranging from 1.0 to 2.4 cm/sec. Based on these results, two pervious concrete mixes with high compressive strength and adequate permeability (comparable to aggregate piers) were selected to cast the piles for the vertical load tests. Both mixtures used a 0.21 water/cement ratio, an 11% sand/aggregate ratio, 377 kg/m³ cement and 1440 kg/m³ coarse aggregate. Test units 1 and 2 used crushed Nazareth aggregate (locally available aggregate in eastern Pennsylvania) and Test units 3 and 4 used pea river gravel (commercially available at home improvement stores). Both aggregates were washed and sieved, and the portion passing through a 9.5 mm sieve and retained on a No. 4 (4.75 mm) sieve was used (see FIG. 1 c). The pervious concrete mixture used in preparing Test unit 2 had an average porosity of 20%, a permeability of 1.33 cm/sec., a 28-day compressive strength of 18.3 MPa, a split tensile strength of 2344 kPa, and an elastic modulus of 16.2 GPa. The pervious concrete mixture used in preparing Test units 3 and 4 had an average porosity of 12.5%, a permeability of 1.21 cm/sec., a 28-day compressive strength of 22.2 MPa, a split tensile strength of 2337 kPa, and an elastic modulus of 15.4 GPa.

Pervious concrete samples cut from Test units 3 and 4 were used to measure the porosity and permeability of these piles. The average porosity and permeability of the precast pile (Test unit 3) were 13.6% and 1.35 cm/sec., respectively. For the installed pile (Test unit 4), the average porosity and permeability were 11.2% and 1.057 cm/sec., respectively. Strength tests were not performed on the samples cut from the test piles because the cutting process affects the strength of pervious concrete as concluded by Suleiman et al. (2006).

By comparing the material properties of aggregate pier and the pervious concrete pile, the following observations can be made: (1) the unconfined compressive strength of the pervious concrete material is more than 10 times greater than that of the confined granular piers; and (2) the permeability coefficient of the pervious concrete piles and granular piers are comparable. Furthermore, according to the analytical work of Han and Gabr (2002) and Suleiman et al. (2003) for embankments supported on several ground reinforcement techniques representing a wide range of modulus ratios (i.e., elastic modulus of pile/elastic modulus of soil), using a pile with a modulus similar to pervious concrete piles will increase the stress concentration ratio by approximately 3 times the ratio for granular piles. This will reduce the stress carried by poor foundation soils and reduce the area replacement ratio, while maintaining the advantages of granular piers in improving the time rate consolidation and reducing liquefaction potential of soils.

Soil Properties

The soil used in all vertical load tests was classified as well-graded sand (SW) according to the United Soil Classification System (FIG. 1 c). The minimum and maximum relative density vibrating table tests [ASTM D4254 (2009e), and ASTM D4253 (2009f)] were performed at oven dry conditions and the minimum and maximum unit weight of the sand were 15.1 kN/m³ and 20.8 kN/m³, respectively (i.e., maximum void ratio of 0.720 and minimum void ratio of 0.250). For each vertical load test, the soil was placed in the large soil box using soil storage and moving system. The dry unit weight and moisture content of the sand placed in the soil box were measured using a nuclear density gauge. The placed soil had an average relative density of 32%, unit weight of 16.5 kN/m³, and water content of 2%. The standard deviation of the unit weight measurements was 0.377 kN/m³, which confirmed the uniformity of the placed soil. (Note: soil placement is described later in the paper). To characterize the soil properties, consolidated drained (CD) triaxial tests were performed. The samples were prepared to achieve a relative density similar to that of the soil in the large soil box (i.e., 32% relative density or unit weight of 16.5 kN/m³). The 70-mm diameter samples were tested at confining stresses of 15, 25, 35, 100 and 160 kPa. The results of the triaxial tests are presented in FIG. 1 d. The K_(f) line presented in FIG. 1 e indicates that the peak friction angle of the soil equals to 38°. The initial modulus of the soil (E_(i)) as a function of confining pressure (σ₃) was evaluated using the power function suggested by Janbu (1963) [i.e., E_(i)=k Pa (σ₃/Pa)^(n), where Pa is the atmospheric pressure, k is the modulus number and n is the modulus exponent] and the calculated values of k and n were 82.3 and 0.95, respectively, as shown in FIG. 1 d.

Testing Facility

The used experimental Soil-Structure Interaction (SSI) Facility had a reaction frame system, advanced sensors, state-of-the-art instrumentation and data acquisition and control system. The two stacked soil boxes and the vertical reaction frame test configuration are shown in FIG. 2. The two soil boxes have dimensions of 1.5×1.5×1.5 m and 1.5×1.5×0.75 m and were designed to allow for flexible assembly.

The advanced sensors available in the SSI facility include customized flexible Shape Acceleration Arrays (SAA) deformation sensors, in-soil null pressure sensors, and tactile pressure sheets. The SAAs consists of a linked series of micro-machined electromechanical sensors that enabled gravity-based shape calculation over the sensed area. The sensors can measure three-dimensional (3D) movement based on a reference point. The SAAs were specially machined with segment lengths of 90 and 120 mm to fit the scale of the performed experiments. The in-soil null pressure sensors were designed in-house with a diameter of 42 mm and a thickness of 7 mm. Each null pressure sensor has an air pressure chamber with an embedded strain gauge. The null pressure sensors are connected to a closed loop system that controls the flow of air to maintain the strain measurement at zero. Talesnick (2005) and Talesnick et al. (2008) tested similar sensors within different soil types at several levels of pressure in a calibration chamber and reported that the difference between the measured and the applied pressure was smaller than 0.3 kPa. The tactile pressure sensor sheets consist of a matrix of small point sensing cells (32×32 sensors) that provide discrete pressure measurements. Palmer et al. (2009) concluded that the accuracy of pressure measurements using the tactile pressure sheets was higher than 90%. Suleiman et al. (2013) studied the accuracy of the pressure measurements obtained in a lateral load test by comparing the applied load to the soil reaction obtained from measured pressures, which resulted in a difference smaller than 8%. The data acquisition and control system combines testing control and sensor monitoring. The system monitors several types of sensors, including load cells, strain gauges, null pressure sensors, tilt meters and displacement transducers. The SSI facility also has a soil storage and moving system, vibrating table to characterize granular material compaction properties, nuclear density gauge, and web broadcasting capability.

Test Units and Instrumentation

Installation of Test Units

A laboratory installation method was developed by the research team to simulate a field construction method. Details of several stages of the laboratory installation method are shown in FIG. 3. The developed installation system consists of a hollow steel mandrel with a specially designed cone at the tip. The mandrel can be vibrated into soil using an attached concrete vibrator (FIG. 3 a) or a Rhino pile driver placed on top of the mandrel. A bracing system was designed to ensure the verticality of the mandrel driving (FIGS. 3 b and c). During mandrel advancement (mandrel penetration stage), the cone tip stays closed. Once the desired depth is reached, the pervious concrete or aggregate is placed inside the mandrel from the top. Then, the mandrel is lifted upward (mandrel retrieval stage) at a slow rate (FIG. 3 d). During the mandrel retrieval stage, the cone tip opens and the pervious concrete or aggregate fills the created space (FIG. 3 e). Two mandrels with outside diameters of 76 mm and 102 mm were designed and fabricated. Test units 1, 2, and 4 (one aggregate pier and two pervious concrete piles) were installed using this installation method, while Test unit 3 was a precast pervious concrete pile.

Description of Test Units

Test units 1 and 2 were cast using the 76-mm diameter mandrel with an embedded length of 864 mm. Test unit 1 was an aggregate pier and Test unit 2 was a pervious concrete pile; both used the same aggregate (Nazareth crushed aggregate) and were installed using the developed laboratory installation method described above. Test units 3 and 4 were pervious concrete piles, which were prepared using pea river gravel, with an embedded length of 1219 mm. Test unit 3 was a precast pile with a 102 mm diameter, which was placed vertically into the soil box and the soil was rained around it. Test unit 4 was installed using the 102-mm diameter mandrel. Since the cone did not completely open during the installation of Test unit 4, the installed pile has a slightly tapered tip with a cross-sectional area of 4825 mm² and an average cross-sectional area along the pile length of 5935 mm².

Instrumentation of Test Units and Surrounding Soil

As mentioned previously, two soil boxes were stacked on top of the other, which provided a total height of 2.25 m, and a reaction frame was assembled for the vertical load tests (FIG. 2 a and FIG. 4 a). To produce a uniform soil, a soil storage and movement system was designed. This system consists of a bottom dump soil container with an attached sieve to rain the soil into the soil box. As shown in FIG. 4 b, the soil was placed in the soil boxes by raining the soil from a height of approximately 1.5 m.

Test unit 1 (aggregate pier) and Test unit 2 (pervious concrete pile) were installed to assess the performance of the developed installation method and to compare the response of the aggregate pier to that of the pervious concrete pile when subjected to vertical loading. These two Test units and the surrounding soil were not reinforced or instrumented and only the vertical applied load and displacement of the pile head were monitored during the load tests.

Test unit 3 (precast pervious concrete pile) and Test unit 4 (pervious concrete pile installed using the developed installation method or simply referred to as installed pervious concrete pile) were used to evaluate the effect of installation on the pile response when subjected to vertical loading. Test unit 3 was reinforced with a No. 4 (12.7 mm diameter) rebar instrumented with strain gauges and placed at the center of pile cross-section, as shown in FIG. 5 a. To evaluate the effects of the soil box boundaries on the installation and response of the pile, one tactile pressure sheet was mounted on the bottom of the soil box and another sheet on the side of the soil box (FIGS. 5 a and b).

Test unit 4 was installed using the developed installation method. Similar to Test unit 3, Test unit 4 was reinforced with a No. 4 rebar with mounted strain gauges along the length of the pile. The surrounding soil was instrumented with three SAAs and six null pressure sensors as shown in FIGS. 4 c, d and FIG. 5. Two of the SAAs were installed at a distance of one pile diameter (1D or 102 mm) from the center of the pile and one SAA was installed at a distance of 2D from the center of the pile (203 mm). As shown in FIG. 5, null pressure sensors 1, 2 and 3 were installed at 76, 178, and 279 mm (i.e., 0.75D, 1.75D, and 2.75D) below the tip of the pile. Null pressure sensors 4, 5 and 6 were installed at a horizontal distance of 1D from the center of the pile at either a depth of 914 mm or 1270 mm below the soil surface. Null pressure sensors 4 and 5 were installed at the same depth with a similar distance from the center of the pile to check the repeatability of the measured stress changes. Furthermore, one pressure sheet was mounted at the bottom of the soil box and one on the side wall of the soil box (FIGS. 5 a and b) to assess the effect of the soil box boundaries during installation and loading. The pressure sheets mounted on the side and at the bottom of the soil box recorded a maximum pressure change of 0.3 kPa and 1.4 kPa, respectively. These measurements illustrate that soil box boundaries has minimal or no effect on pile installation and soil-pile system response when subjected to vertical loading. It should be noted that several strain gauges used in the installed pile (Test unit 4) may have been damaged during installation and did not function properly during the vertical load test.

Loading Sequence

The four vertical load tests were conducted in general accordance with the fast procedure outlined in the ASTM D1143 (2009h). Each load level was held constant for at least 4 minutes or until the pile head displacement stabilized. The test was stopped when the pile displacement continued increasing without an increase in the applied load. During testing of Test units 1 and 2, a load increment of 222.4 N was used. For Test units 3 and 4, load increments of 222.4 N and 889.6 N were used.

Test Results

Experimental Pile Load-Displacement Response

Effect of Pile Type on Response

FIG. 6 a presents the measured vertical load-displacement responses for Test unit 1 (aggregate pier) and Test unit 2 (pervious concrete pile). Both test units were made using the same aggregate and installed using the same installation method. The ultimate load of Test unit 1 was 2.2 kN and the ultimate load of Test unit 2 was 9.8 kN. Therefore, the ratio of the ultimate load of the pervious concrete pile to the ultimate load of the aggregate pier was 4.4. After the ultimate load was reached during testing, the soil surrounding each pile was removed to expose the Test units. FIG. 7 a illustrates that Test unit 1 (aggregate pile) failed by bulging into the surrounding soil due to the low confining pressure provided by the surrounding soil. The depth of the bulged zone was approximately 2.5D below the soil surface. For Test unit 2 (pervious concrete pile), the pile failed by punching vertically into the soil (FIG. 7 b). This type of failure indicates that the load-displacement response of the pervious concrete pile was controlled by the soil-pile interaction along the pile length and the soil resistance at the pile tip, not by the confinement provided by the surrounding soil.

Effect of Pile Installation Method on Response.

FIG. 6 b presents the measured vertical load-displacement responses for Test unit 3 (precast pervious concrete pile) and Test unit 4 (installed pervious concrete pile). These tests were conducted to compare the load-displacement response of two similar pervious concrete piles that were installed using different methods. The ultimate load of Test unit 3 was 12.2 kN and the ultimate load of Test unit 4 was 31.2 kN. Therefore, the ratio of the ultimate load of the installed pervious concrete pile to the ultimate load of the precast pile was 2.6. The difference between the ultimate loads of the two pervious concrete piles occurs because of the installation method, which has significant effects on the surrounding soil properties that will be briefly discussed later in this paper. Similar to the failure experienced by Test unit 2, Test units 3 and 4 also experienced vertical punching failures as shown in FIGS. 7 c and d.

Load Transfer along Pile Length

Using the strain gauge measurements along the pile length and the calculated modulus of the pervious concrete composite section, including the steel reinforcing bar, the load transfer along the pile length for Test units 3 and 4 was calculated and compared at loading stages of 1.78 kN, 4.89 kN and 11.12 kN (FIG. 8). These loading stages were selected for comparison because they represent the initial (linear) stage, transition stage and near the ultimate load for Test unit 3. The rate of load transfer shown in FIG. 8 indicates that at the applied load of 11.12 kN, the maximum unit friction, which is the slope of the curve between depths of 381 mm and 965 mm, was 9.0 N/mm for Test unit 3 (precast pile) and 10.6 N/mm for Test unit 4 (installed pile). These unit friction values illustrate that the pile installed using the developed installation method had a 17.8% higher load transfer rate to the surrounding soil through shaft resistance than the precast pile at this loading step. Extending the load transfer curves in FIG. 8 to the depth of the pile tip for the 11.12 kN loading stage results in tip resistances of 1.102 kN and 3.278 kN for Test units 3 and 4, respectively. At the ultimate load of Test unit 4 (31.2 kN), the tip resistance was approximately 35% of the applied load (i.e., shaft friction resisted 65% of the applied load). The difference in the ultimate load and load transfer between Test unit 3 and 4 is mainly due to the used installation method, which for the installed pile changes the soil density and soil stresses, as well as results in a rougher pile surface as shown in FIG. 7 c and d.

Using the load transfer along the pile length and displacements back calculated using the strain measurements along the pile, the shear stress-displacement relationships at the soil-pile interface (i.e., t-z curves) were developed and are presented in FIG. 9. The results illustrate that Test unit 4 (installed pile) had a higher maximum shear stress transfer than that of Test unit 3 (precast pile). The ratio of maximum shear stress at the soil-pile interface for Test unit 4 relative to that of Test unit 3 was 2.5 at an average depth of 191 mm below the soil surface and 5.3 at an average depth of 635 mm. These ratios are consistent with those reported in the literature when comparing displacement and non-displacement piles (e.g., Colombi et al., 2006). The differences in ultimate load and load transfer clearly illustrate the significant effect that the installation method had on the soil-pile interaction for vertically loaded pervious concrete piles.

Effects of Pile Installation on Surrounding Soil

As the mandrel penetrates the soil, it pushes the soil downward and laterally (cavity expansion) resulting in a significant increase of vertical stress and a smaller increase of horizontal stress (Basu et al., 2011). For a soil element at a specific depth, the cavity expands until the mandrel, which has a constant diameter, starts to penetrate the location of the soil element. Vertical shearing is then applied to the soil element as the mandrel penetrates deeper. As discussed before, the effects of pile installation on soil lateral displacement (movement) and stresses were monitored using SAAs and null pressure sensors for Test unit 4. The in-soil null pressure sensors were zeroed before starting the experiment which did not allow us to record the initial horizontal stresses (i.e., allowing the measurement of the change of stresses during installation and load testing). Null pressure sensor 2 did not function properly due to air leakage during installation.

Lateral Soil Movement

The lateral soil movement due to pile installation measured using the SAAs are summarized in FIG. 10. The SAAs measurements at 1D from the center of the pile, which was 51 mm from the surface of the pile, illustrate that the steel mandrel penetration resulted in a non-uniform lateral soil movement along the depth of the mandrel. The lateral movement near the soil surface was 17.6 mm, while the lateral movement near the pile tip was 8.2 mm. Therefore, the soil movement at the pile tip is only 50% of the soil movement near the soil surface. This is contrary to the assumption of uniform soil displacement along the depth of the pier commonly used by researchers numerically investigating the effects of aggregate piers installation on the surrounding soil (e.g., Chen et al., 2009; and Dijkstra et al. 2011). During vertical loading, the soil at 1D experienced 1.7 to 5.8 mm additional lateral movement. At a distance of 2D (152.4 mm from the surface of the pile), the lateral soil movement was 4.1 mm at the soil surface with no lateral soil movement occurring below a depth of 500 mm (−5D).

Horizontal Soil Stresses

The changes of horizontal soil stresses due to pervious concrete pile installation and during vertical loading, which were measured using null pressure sensors 4, 5 and 6, are summarized in FIG. 11 a. The following observations can be made regarding the changes occurring in the horizontal stresses: (1) due to mandrel advancement, the change of horizontal stresses measured by sensor 6 was 147 kPa at 914 mm below the soil surface; (2) the horizontal stress increased by 99 kPa at 1240 mm below the soil surface (i.e., near the tip of the pile); (3) sensors 4 and 5, located at the same depth and distance from the pile, showed similar horizontal stress increases during mandrel advancement, which confirms the repeatability of the stress measurements for the sensors.; (4) during the vertical load test, the horizontal stresses measured by sensor 6 increased, which is similar to the trend reported by Lehane et al. (1993); and (5) during the vertical load test, Test unit 4 with a slightly tapered tip (see FIG. 5) penetrated the soil below the pile tip resulting in cavity expansion at the location of sensors 4 and 5 approaching a condition similar to that at the location of sensor 6. Therefore, it was expected that at this stage the change of soil horizontal stress measured using sensors 4 and 5 would be similar to that of sensor 6, which is observed in FIG. 11 a. This result is another confirmation of the repeatability of the measured stresses.

The measured changes of soil horizontal stresses were compared with the elastic cavity expansion solution presented by Yu (2000). The calculated change of horizontal stress at the location of sensor 6 (914 mm below soil surface) was 114.8 kPa, which is approximately 20% smaller than the measured change in soil horizontal stress at the same location (145 kPa). At the location of sensors 4 and 5 (depth of 1240 mm), the calculated change in soil horizontal stress was 172.3 kPa, which is approximately 17% smaller than the average measured pressure using sensors 4 and 5. Further analysis is currently being conducted by the research team, which will be presented in another paper.

Vertical Soil Stresses

For changes in vertical stresses, which were measured using sensors 1 and 3, the vertical stress increased by 180 kPa at 76 mm below the tip of the cone due to mandrel advancement. The increase of vertical stress is approximately 1.9 times the increase in horizontal stress measured using sensors 4 and 5 (located near sensor 3). The result of having the vertical stress significantly higher than horizontal stress during cone advancement was also reported by Salgado et al. (1997) and Salgado and Prezzi (2007). Sensor 1 showed an increase of vertical stress of 57 kPa at the end of mandrel advancement. At the end of the load test, sensor 3, which was approximately 55 mm below the pile tip at this stage, recorded an increase of vertical stress of 638 kPa and sensor 1, which was approximately 60 mm below the pile tip at this stage, recorded an increase of 135 kPa (FIGS. 11 a and b).

FIG. 11 b summarizes the development of measured pressures from sensor 6 during different stages focusing on mandrel advancement stage. During mandrel advancement, the change of horizontal stress measured using sensor 6 increased until the mandrel passed the location of sensor 6. The horizontal stress measured by sensor 6 then decreased as the mandrel advanced deeper. This trend is consistent with results reported by several researchers including Lehane and White (2005) and Basu et al. (2011).

The open structure (permeability) of the pervious concrete allows the ground water to penetrate the concrete and be in direct contact with closed loop pipes. This improves the heat transfer between the loops and the surrounding soil.

The pervious concrete mixtures developed by other researchers are stiff and not easy to flow for a field installation. For foundation applications, however, the workability (how easy to flows) of the concrete is vital for the installation process. Therefore, there is a need to develop a workable or flowable pervious concrete. The workability of the concrete is measured using a test called the slump test. The slump of pervious concrete is not commonly reported in the literature because it is usually very small (less than 1 or 2 inch). As part of this research effort, a pervious concrete mixture with workability slump of more than 6 inches was developed. The following Table show the quantities used in the developed mix for field application.

TABLE 2 Aggregate Water Cement Water/Cement Aggregate type Size lbs/ft³ lbs/ft³ ratio Delaware Pea No sieved 7.53 23.54 0.32 gravel AEA Super plasticizer Sand Aggregate Sand/Aggregate ml/100 lbs ml/100 lbs lbs/ft³ lbs/ft³ ratio cement cement 0 100 0 88.8 444

The properties of the pervious concrete developed for field applications are as follows: slump of 6-8 inch; 7-day compressive strength of 2227 psi; and permeability of 0.66 cm/sec.

To develop a geothermal pervious concrete pile, installation, tests and analyses were conducted. The results show than the thermal conductivity of pervious concrete in water (simulating the case for closed loop system), the thermal conductivity of the pervious concrete was 30% to 100% higher than the normal concrete where the average thermal conductivity of normal concrete range from 1.5-2.5 W/mK and the pervious concrete in water was 3.27 W/mK. For the case of water flowing through the pervious concrete (open loop system), the thermal conductivity was at least 80% higher than the normal concrete (4.5 W/mK vs. 2.5 W/mK). Based on preliminary simplified analyses, these results indicate that the improvement of the heat transfer in the system range from ˜10% to 25%

Fuller description of Drawings. FIG. 1( a) is a photographic depiction of pervious concrete samples prepared by the methods described herein in accordance with the present invention.

FIG. 1( b) is a graph illustrating pervious concrete compressive strength and permeability samples prepared by the methods described herein in accordance with the present invention.

FIG. 1( c) is a graph illustrating gradation of aggregate used for casting test piles in examples herein, as well as for soil used in the soil box herein in accordance with the present invention.

FIG. 1( d) is a graph illustrating CD triaxial tests on sand samples in examples herein in accordance with the present invention.

FIG. 1( e) is a graph illustrating a diagram showing p′_(f)-q′_(f) for the peak stresses of soil samples in examples herein in accordance with the present invention.

FIG. 2( a) is a schematic drawing illustrating a top perspective view of the SSI facility and reaction frame system for vertical load tests in accordance with the present invention.

FIG. 2( b) is a photograph illustrating a side elevational view of the top box and reaction system of the SSI facility in accordance with the present invention.

FIGS. 3( a)-(e) are a series of photographs collectively depicting an example of the pervious pile installation method steps in an example herein; wherein, FIG. 3( a) is a photograph illustrating an assembly of a cone, mandrel, and vibrator in accordance with the present invention; FIG. 3( b) is a photograph illustrating an assembly of a bracing system assembled in the SSI facility top soil box in accordance with the present invention; FIG. 3( c) is a photograph illustrating an assembly of the cone, mandrel, and vibrator of FIG. 3( a) in conjunction with the bracing system of FIG. 3( b), the mandrel being inserted and driven into the soil box in accordance with the present invention; FIG. 3( d) is a photograph illustrating a step of casting a pile using the inserted and driven assembly of a cone, mandrel, and vibrator in accordance with the present invention; and FIG. 3( e) is a photograph illustrating the effect of opening the cone tip of FIG. 3( a) during the casting step of FIG. 3( d) vibrator in accordance with the present invention;

FIGS. 4( a)-(d) are a series of photographs collectively depicting an example of the vertical load test setup and results in example herein; wherein: FIG. 4( a) is a photograph illustrating an exemplary assembly of a top soil box, bottom soil box, and reaction frame in accordance with the present invention; FIG. 4( b) is a photograph illustrating an exemplary setup and method step of raining soil into a bottom soil box from an elevation of about 1.5 meters in accordance with the present invention; FIG. 4( c) photograph illustrating an exemplary assembly and arrangement of SSAs used in experimental examples for forming piles in accordance with the present invention; and FIG. 4( d) is a photograph illustrating an exemplary assembly and arrangement of in-soil null pressure sensors used in experimental examples for forming piles in accordance with the present invention. FIG. 4( e) is an enlarged view taken from FIG. 4( d) showing a null pressure sensor.

FIG. 5( a) is a schematic side cross-sectional view of the test SSI facility assembly used in performing experimental methods for Test Unit 4 to create experimental pile apparatus in accordance with the present invention.

FIG. 5( b) is a schematic top cross-sectional view of the test SSI facility assembly used in performing experimental methods for Test Unit 4 to create experimental pile apparatus in accordance with the present invention.

FIG. 6( a) is a graph depicting vertical load versus displacement for Test Units 1 and 2 for creating 76 mm diameter piles in accordance with the present invention;

FIG. 6( b) is a graph depicting vertical load versus displacement for Test Units 3 and 4 for creating 102 mm diameter piles in accordance with the present invention.

FIGS. 7( a)-(d) are a series of photographs collectively depicting the results of pervious pile installation methods in examples herein, wherein; FIG. 7( a) is a photograph illustrating a result in Test unit 1, showing a bulging failure of the pile; FIG. 7( b) is a photograph illustrating a result in Test unit 2, showing a pile assembly in accordance with the present invention in accordance with the present invention; FIG. 7( c) is a photograph illustrating a result in Test unit 3, showing a pile assembly in accordance with the present invention; and FIG. 7( d) is a photograph illustrating a result in Test unit 4, showing a pile assembly in accordance with the present invention.

FIG. 8 is a graph illustrating a comparison of the force transferred along the length of piles as described in Test unit 3 and Test unit 4 for different loading stages, in accordance with the present invention.

FIG. 9( a) is a graph illustrating shear stress versus displacement curves (t-z curves) for the soil-pile interface calculated using the strain gauge measurements for Test units 3 and 4 for average depth below soil surface of 191 mm, in accordance with the present invention.

FIG. 9( b) is a graph illustrating shear stress versus displacement curves (t-z curves) for the soil-pile interface calculated using the strain gauge measurements for Test units 3 and 4 for average depth below soil surface of 635 mm, in accordance with the present invention.

FIG. 10 is a graph illustrating soil lateral displacement at 51 mm and at 152 mm from the surface of the pile measured using SSAs for Test unit 4, in accordance with the present invention.

FIG. 11( a) is a graph illustrating the effect of installation on the change of soil pressure showing a summary of pressure changes developed at the end of installation and during load testing, in accordance with the present invention.

FIG. 11( b) is a graph illustrating the effect of installation on the change of soil pressure showing details of changes of horizontal pressure measured using sensor 6 and changes of vertical pressure measured using sensor 3 focusing on the changes during installation, in accordance with the present invention.

SUMMARY AND CONCLUSIONS

A new ground improvement system method has been developed using pervious concrete energy piles. The material properties of the pervious concrete and the response of different vertical load tests were garnered using the SSI Facility. Initially, Test unit 1 (aggregate pier) and Test unit 2 (pervious concrete pile) were installed to investigate the effectiveness of the designed laboratory installation method and to compare the vertical load-displacement response of the aggregate pier to that of the pervious concrete pile. Then two fully-instrumented vertical load tests were performed on a precast pervious concrete pile (Test unit 3) and an installed previous concrete pile (Test unit 4) to evaluate the effects of the installation method on the soil-pile interaction.

Based on the experimental results obtained from the four vertical load tests and the discussion of the results presented herein, the following conclusions were made:

Pervious concrete piles have a compressive strength that is more than 10 times that of granular piers, while providing similar permeability to granular piers.

The pervious concrete pile (Test unit 2), which had the same dimensions, aggregate type, and installation method, as the aggregate pier (Test unit 1), had an ultimate load that was 4.4 times greater than the ultimate load of the aggregate pier.

Furthermore, the pervious concrete pile failed by vertically punching into the soil at the pile tip, while the aggregate pier failed by bulging outward into the surrounding soil.

The installation method had significant effects on the response of the pervious concrete piles. When comparing the response of the two pervious concrete piles installed using different methods [precast pile (Test unit 3) and installed pile (Test unit 4)], the ultimate load of the installed pile was 2.6 times greater than the ultimate load of the precast pile.

Installation of the pervious concrete pile resulted in an increase of the maximum shear stress transferred at the soil-pile interface. The ratio of the maximum shear stress calculated using the strain gauges for the installed pile compared to the precast pile ranged from 2.5 to 5.3.

The lateral soil displacements measured at a distance of 1D from the pile center during installation were not uniform along the length of the pile. The installation of the pile also resulted in significant increases in the soil vertical stress and a smaller increase in the soil horizontal stress. The measured change of the vertical and horizontal soil stresses showed trends similar to those reported in the literature. In further field applications, the following mixture was developed and utilized as shown in Table 2.

TABLE 2 Aggregate Water Cement Water/Cement Aggregate type Size lbs/ft³ lbs/ft³ ratio Delaware Pea No sieved 7.53 23.54 0.32 gravel AEA Super plasticizer Sand Aggregate Sand/Aggregate ml/100 lbs ml/100 lbs lbs/ft³ lbs/ft³ ratio cement cement 0 100 0 88.8 444

Other modifications may also be made to the apparatus and method described above without departing from the scope of the invention as defined in the following claims. 

1. A method of installing a pervious concrete pile, the method comprising the steps of: a. Providing a mandrel, the mandrel having a cone-shaped tip on a distal end thereof and an inlet opening on the opposite end thereof, a mandrel body connecting the inlet opening to the cone-shaped tip, the cone tip having means for opening and closing of the cone tip to allow material to pass from the inlet opening of the mandrel, through the mandrel body, and out of the con-shaped tip when the cone-shaped tip is opened; b. Providing a vibration mechanism in vibrational contact with the mandrel; c. Inserting the mandrel into a geological formation at a preselected location; d. After installing the mandrel into the geological formation at a preselected location, delivering a selected amount and type of aggregate and concrete mixture into the inlet opening of the mandrel; e. Operating the vibration mechanism for a time sufficient to ensure delivery of the mixture to the cone-shaped tip of the mandrel; f. Opening the cone-shaped tip of the mandrel to allow at least some of the mixture to escape from the tip to form a pervious concrete pile adjacent the exterior of the cone-shaped tip; and g. Withdrawing the mandrel from the geologic formation at a rate sufficient to form a continuous pervious concrete pile within the geologic formation, the continuous pervious concrete pile having a porosity of between about 10% to about 40%, and a permeability coefficient of between about 0.01 to about 2 cm/sec.
 2. The method of claim 1, further comprising the step of providing a heat exchange apparatus surrounded by the mixture of the continuous pervious concrete pile.
 3. The method of claim 2, further comprising the step of operating the heat exchange apparatus to permit the exchange of heat between the geologic formation and a heat exchange medium controlled by the heat exchange apparatus.
 4. The method of claim 3, further comprising the step of providing a bracing apparatus to guide the insertion of the mandrel into the geologic formation at the preselected location.
 5. The method of claim 1, wherein the aggregate and concrete mixture comprises a water to cement ratio of between about 0.2 and 0.50.
 6. The method of claim 1, further comprising the step of providing at least one geothermal heat exchange apparatus in thermal communication with the pervious concrete pile.
 7. The method of claim 6, wherein the geothermal heat exchange apparatus comprises at least one fluid loop that becomes surrounded by the mixture and becomes an integral part of the pervious concrete pile.
 8. The method of claim 7, wherein the geothermal heat exchange apparatus further comprises an open fluid loop that is in thermal exchange communication with the geothermal formation.
 9. The method of claim 7, wherein the fluid loop is in heat exchange connection with a heat exchange system of a building supported by the pervious concrete pile.
 10. The method of claim 1, wherein the step of inserting the mandrel into a geologic formation does not require a separate step of drilling a hole into the formation before mandrel insertion.
 11. The method of claim 1, wherein the pervious concrete pile exhibits a vertical load capacity of at least 2 times greater than that of a granular pier having the same cross-sectional diameter.
 12. The method of claim 1, wherein the pervious concrete pile exhibits a vertical load capacity of at least the same capacity of a prefabricated concrete pier having the same cross-sectional diameter.
 13. The method of claim 1, wherein the thermal conductivity of the pervious concrete is greater than about 2.6 W/mK.
 14. A geothermal heat exchange system comprising at least one pervious concrete pile having a porosity of between about 10% to about 40%, and a permeability coefficient of between about 0.01 to about 2 cm/sec.
 15. The system of claim 14, wherein the thermal conductivity of the pervious concrete is greater than about 2.6 W/mK.
 16. The system of claim 14, further comprising a heat exchange apparatus in thermal communication with the continuous pervious concrete pile.
 17. The system of claim 16, wherein the heat exchange apparatus comprises at least one fluid loop surrounded by the mixture of the pervious concrete pile, the at least one fluid loop in thermal communication with the geologic formation.
 18. The system of claim 17, wherein the fluid loop includes at least one closed fluid loop that is in thermal exchange communication with the geothermal formation.
 19. The system of claim 17, wherein the fluid loop includes at least one open fluid loop that is in thermal exchange communication with the geothermal formation.
 20. The system of claim 14, wherein the pervious concrete pile exhibits a vertical load capacity of at least 2 times greater than that of a granular pier having the same cross-sectional diameter. 